1. Field of the Invention
This invention relates to thermosetting resin compositions for overcoats of flexible circuits, which are particularly excellent in low shrinkage during curing and flexibility.
2. Prior Art
Formerly, the surface protective films of flexible wiring circuits include, for example, those prepared by cutting polyimide films, called as coverlay films, using a die made corresponding to the pattern, which are then adhered onto a substrate with an adhesive; and those prepared by applying a ultraviolet rays--curable or thermosetting overcoat composition the film from which is imparted with flexibility, according to the screen printing method, the latter being more advantageous in workability.
As such curable overcoats, resin compositions mainly consisting of an epoxy resin, an acryl resin, or a mixture thereof have been known. These resins often comprise, as main ingredients, a resin denatured by introducing, particularly, a butadiene or siloxane skeleton, or a long chain fatty skeleton, thereby improving flexibility and preventing warp due to cure shrinkage while trying to avoid reduction of inherent heat resistance, chemical resistance, and electrical insulation.
Recently, however, with lightening and miniaturization of electronic equipment (cameras, 8 mm automatic projectors, portable telephones, personal computers, printers, etc.), flexible substrates have been reduced in thickness and weight, and thus more remarkably affected by flexibility and cure shrinkage of the cured resin composition to be overcoated. Therefore, curing type overcoats do not satisfy requirements for warp upon cure shrinkage and flexibility of the cured products.
[Problems to be Solved by the Invention]
In consideration of such points, there have been found no curable resin compositions having properties required of overcoats for flexible circuit such as low shrinkage during curing, and whose cured products have satisfactory properties such as flexibility, adherence, electrical insulation, chemical resistance, heat resistance, etc., and therefore development of such compositions has been expected.